It has been known that cellulose derivatives inclusive of hydroxypropyl cellulose (HPC) and cellulose triphenylcarbamate form a chiral nematic (cholesteric) liquid crystalline phase by being dissolved in a proper solvent under appropriate conditions, e.g. temperature, concentration. The chiral nematic liquid crystalline phase has a structure where sheet-like nematic liquid crystal phases are piled helicoidally, with each rotating in the direction of orientation. The chiral nematic liquid crystalline phase shows a specific optical properties: the phase selectively reflects circularly-polarized light which corresponds to the orientation direction of a sheet-like nematic liquid crystal. In this selective reflection, the chiral nematic liquid crystal reflects, at the maximum, the wavelength which corresponds to the product of the cycle (pitch) at which the orientation of the sheet-like nematic liquid crystal rotates in 360 degrees and the average refraction index of the liquid crystal phase. It is reported in detail, for example, that such a selective reflection of the chiral nematic liquid crystal is useful for laser systems as a part of a circularly polarized light-generator [see, for instance, Jacobs, Journal of Fusion Energy, 5(1), 65-75 (1986)]. If the wavelength of the selective reflction by the chiral nematic liquid crystal lies within a visible light region, the chiral nematic liquid crystal develops a beautiful colour, thus being useful for applications as decorative materials.
However, formation of the chiral nematic liquid crystal is strictly governed by the conditions such as temperature and concentration. Besides, the chiral nematic liquid crystal has to be enclosed or sealed between glass plates in a liquid state. For these reasons and others, the chiral nematic liquid crystal has limited applications and scarcely used on an industrial basis.
To find a solution to this problem, attempts have been made to give a solid structure to the chiral nematic liquid crystalline phase by solidifying a polymer solution containing the chiral nematic crystalline phase. Charlet and Gray prepared a film which selectively reflects right-circularly polarized light by removing water from an HPC aqueous solution [Macro-molecules, 20, 33-38 (1987)]. However, its reflection intensity is about 0.8 to 1.5 relative to 20 to 30 .mu.m film thickness, in terms of the absolute value of ellipticity of the circular dichroism spectrum as the index. Such a reflection intensity is unsatisfactory in practice. Besides, since HPC is soluble in water, its applications or the manner of its use is limited.
Charlet and Gray reported three process for a film preparation which shows a selectivity in reflection: (i) a method comprising removing a solvent from an isotropic solution of hydroxypropyl cellulose; (ii) a method which comprises putting a liquid crystal solution of hydroxypropyl cellulose between glass plates, forming a liquid crystal phase by sliding or shifting one of the glass plates, keeping the phase under its own vapour pressure for a few minutes to eliminate the orientation by shearing, and thereafter removing the solvent; and (iii) a method which comprises putting the HPC solution between porous materials and removing the solvent.
Nevertheless, the above methods cannot provide a solid film which has reflection selectivity, in the case of the chiral nematic liquid crystal system composed of a number of cellulose derivatives (e.g. halogenated hydrocarbon solutions of ethylcellulose, phenol solutions, or organic acid solutions). This is presumably because of imperfect formation of liquid crystal structure, deviation of the wavelength of the selective reflection from a visible light region, or deformation of the liquid crystal structure due to the shearing forced generated during preparation. Although this report mentions that the pitch of the liquid crystalline phase in the solid product depends on the drying rate of the solvent, no methods are suggested for preparation of a selectively reflective film. As a matter of fact, it is very rare, even counting the report of Charlet et al., that a film with reflection selectivity is prepared from cellulose derivatives by a so-called casting method, i.e., a method of removing a solvent from a polymer solution. In general, it is very hard to achieve a successful preparation of a reflection-selective film by the casting method.
Suto et al. prepared a film which shows positive circular dichroism at 200 to 300 nm by thermal compression moulding ethyl cellulose with degree of substitution (DS) of 2.67, or by removing a solvent from an m-cresol solution of the ethyl cellulose in a normal laboratory atmosphere [J. Appl. Polym. Sci., 61, 1621 (1996)]. However, the product has a short wavelength of the selective reflection, exhibiting no selective reflection in a visible light region. Suto et al. do not state any method for controlling the wavelength of the selective reflection of the products. Further, the selective reflection intensity is unclear, as they fail to indicate the unit of the circular dichroism spectrum.
Watanabe et al. prepared a film which selectively reflects left-circularly polarized light by the casting method from a polyglutamic acid derivative which forms the chiral nematic liquid crystal [Polymer Journal, 9(3), 337-440(1977)]. The intensity of the selective reflection is unknown because of a lack of quantitive descriptions. It should be borne in mind that the polymer (polyglutamic acid derivative) is either hard to obtain or expensive. This literature mentions the influence of a solvent or a solvent mixture on the wavelength of selective reflection of a film. Nevertheless, no methods are disclosed as to the preparation of a selectively reflective film.